A fundamental consideration in any telecommunications system design is switching capacity. Switching capacity must be analyzed in terms of current demand and projected demand in order to find a solution that is cost effective for both present and future service. For example, assume that a developing country is in the process of building a basic telecommunications system and intends to provide service to most of its current population. Such a population is most likely geographically distributed among small areas of high density (cities) and larger areas of low density (suburban and rural). In addition, the population is probably growing, but at different rates in different areas. Thus, the challenge for a telecommunications system designer is to provide sufficient switching capacity to support satisfactory service to most or all of the population while also anticipating likely increases in future demand and providing for economical expansion.
Another example of the difficulty of providing appropriate switching capacity involves wireless or personal communications network (PCN) applications. These types of applications are based on micro-cellular architectures which require numerous base-stations, in close physical proximity across a metropolitan area, with different switching capacities which aggregate to a large capacity.
A second fundamental consideration in telecommunications system design is providing for the addition of new features or services in the future. Telecommunications equipment and service continues to evolve rapidly, due in large part to the advent of digital technology. Even more dramatic advances are likely in the future, particularly as previously separate industries such as cable television and local telephone operating companies integrate their services. Again, the challenge is to create a system which economically serves a present need, while also providing flexible and inexpensive ways to integrate new features and services as they become available.
Of the conventional approaches to the dual problems of providing adequate switching capacity along with access for new features and services, most, if not all, suffer from one or both of two major disadvantages: (1) there is insufficient bandwidth in the system to handle information such as video or multimedia (in addition to voice and data), (2) there is no direct, ready access to all of the information passing to or from the system, meaning there is no way to capture all of the information and distribute it to other switching systems or equipment, and (3) an increasingly large central switch is required to provide access to some types of enhanced services.
One conventional approach may be referred to, for shorthand, as the "bus extension" approach. In many conventional telecommunications switches, one or more internal buses are provided for carrying information, including voice, data and control information, between various parts of the switch. Buses are well suited for carrying such information since, by definition, multiple devices (e.g., circuit boards or cards) may interface with the buses and share them in accordance with a defined communication protocol. In a telecommunications switch, it is typical to find one or more buses interconnecting a series of cards which physically terminate telephone lines or trunks with other cards which perform switching, control or other functions.
As the shorthand name suggests, the concept underlying the bus extension approach is simply to connect additional cards, which provide additional switching capacity or other functions, with the existing buses. In addition to the two major disadvantages noted above, there are several other disadvantages to this approach. First, there are physical limitations as to the number of cards that can be physically connected to or share the buses without degrading the system's performance. Second, in order to permit significant future expansion, the buses and other portions of the system must be constructed, in the first instance, to handle far greater traffic than is required prior to any expansion of the system. These limitations are related to the electrical and mechanical characteristics of the buses (or perhaps a particular one of the buses) and their effective operating speeds. Attempts to overcome these limitations (e.g., using an excessively large number of connections to the bus) tends to increase the cost and complexity of the "base" or unexpanded system, possibly rendering the system too costly for some applications. There is also a limitation related to the processing power required to actually performing the switching functions as well as control traffic on the buses.
Third, the bus structures found in many, if not most, conventional switching systems are generally designed solely for carrying out basic call processing and switching functions and do not provide ready, direct access to the ports for integrating new features and services.
Fourth, the bus structures are typically incapable of carrying packet switched data or other types of information.
A second approach may be referred to as the "modular" approach for shorthand. In the modular approach, the concept is to provide a switching system which is constructed from a series of essentially identical modules. Each module provides a finite amount of switching capacity which may be added to an existing system (one or more at a time) to increase the overall capacity of the system.
Again, in addition to the major disadvantages noted earlier, the modular approach has other deficiencies. In order to provide fully non-blocking operation, each and every module as built must have the capability to receive circuit switched data from every other module up to whatever the maximum number of modules may be. In terms of hardware, this means that each module must be built with a sufficiently large memory to hold the maximum amount of circuit switched data which could be received if the maximum number of modules are connected together. For example, if each module is capable of switching the equivalent of 64 ports and a maximum of eight modules may be connected together, then each module must necessarily contain a memory capable of holding circuit switched data for (8.times.64=) 512 ports. Thus, in the modular approach, it is the maximum switching capacity of the fully expanded system which determines the size of the memory that each module must have. For larger systems (i.e., on the order of a few thousand ports or larger), constructing such a memory becomes impractical due to both the accompanying number of physical network/line interfaces as well as the additional circuitry needed to control the memory.
Second, in order to maintain a truly "modular" system, it is impossible to vary the switching capacity of individual modules.
Third, like the bus extension approach, the modular approach is oriented toward performing basic switching operations and does not generally offer direct access to all the ports nor the capability of handling packet switched data or other types of information.